Mercury dashpot timing device



April -7 E. x. sci-MDT El AL 1,955,565

MERCURY DASHPOT TIMING DEVICE Filed March 50, 1951 2 Sheets-Sheet l Aril 17, 1934. E. x. SCHMIDT ET AL MERCURY DASHPOT TIMING DEVICE FiledMarch so, 1951 2 Sheets-Sheet 2 Patented Apr. 17, 1934 UNITED STATESPATENT OFFICE 0. Stevens,

Milwaukee,

Wis, assignors to Cutler-Hammer, Inc., Milwaukee, Wis., a corporation ofDelaware Application March 30, 1931, Serial No. 526,444

14 Claim.

This invention relates to improvements in mercury dash-pot timingdevices.

Dash-pots as heretofore proposed for use in timing the operation ofelectric switches and the like inherently possess certain undesirablecharacteristics which result in irregular timing, such irregularitybeing due primarily to changes in temperature, uncontrollable leakagepast the piston, and the fact that foreign matter gets into thedash-pot.

An object of the invention is to provide a dashpot wherein thedifliculties aforementioned are overcome.

Another objectis to provide a dash-pot having improved structural andoperative characteristics whereby the novel results hereinafterdescribed are attained.

Another and more specific object is to provide a mercury dash-potutilizing a piston and cylinder each of which is molded from a compoundcomprising the phenol-formaldehyde condensation product sold under thetrade-name of "Bakelite the Bakelite" being impregnated with orcontaining graphite.

Other objects and advantages of the invention will hereinafter appear.

The accompanying drawings illustrate an enibodiment of theinventionwhich will now be described, it being understood that theinvention is susceptible of embodiment in other forms without departingfrom the scope of the appended claims.

In the drawings, Figure 1 is an enlarged vertical sectional view of amercury dash-pot timing device constructed in accordance with myinvention,showing the position of the parts at approximately thebeginning of the timing stroke of the piston.

Fig. 2 is a similar fragmentary view, of normal size, showing theposition of the parts at the end of the timing stroke of the piston.

Fig. 3 is a view similar to Figs. 1 and 2, showing the relation of theparts during the up-stroke of the piston for resetting of the timingmechanism.

Fig. 4 is a view, partly in side elevation and partly in verticalsection, of certain parts of a switch mechanism having our improvedtiming device associated therewith.

Fig. 5 is a perspective view, in separated relation, of certain of theparts illustrated in Figs. 1,

2 and 3, and

Fig. 6 is a view partly in end elevation, and partly in vertical sectionon the line 66 of Fig. 4, showing the novel form of bearing for theactuating shaft of the timing device.

Bakelite and graphite of the character dis- Referring to the drawings,the numeral 10 designates a cup or cylinder and 11 designates a pistonslidable within said cup,the cup and the piston each being molded from acomposition of closed in Patent No. 1,182,258, dated May 9, 1918 toFrank Engel.

The numeral 12 designates a body or quantity of mercury the rate of flowof which from each end of the piston 11 to the other controls the rateof movement of said piston in opposite directions within cylinder 10.

By employment of a piston and a cylinder formed of molded Bakeliteimpregnated with graphite the clearance between said parts may be madeextremely small. Therefore by using mercury in the dash-pot as theactive material there will be no leakage past the piston even withrelatively great differences in pressure between the top and bottom ofthe piston. Since no leakage occurs, the dash-pot will have no variationin the coeflicient of discharge; such variation being a common defect inordinary types of dash-pots, due to sidewise or lateral movement of thepiston within the cylinder.

Thus the timing of a mercury dash-pot of the character hereindiscloseddepends primarily on the density of the mercury and issubstantially independent of viscosity. The variation in timing due totemperature changes is therefore negligible, since the density ofmercury changes only approximately two per cent from zero to one hundreddegrees C., and the timing would vary only as the square root of suchchange.

In operation, mercury is forced from the lower end of piston 11 to theupper end thereof through a timing passage 13, 14, 15, 16 and 17 duringthe timing stroke. On resetting movement of the parts of the timingdevice the mercury may pass both through the timing passage 17, 16, 15,14 95 and 13 and through the check valve opening 48, as indicated by thearrows in Fig. 3.

Referring more particularly to the cup or cylinder 10, the same isprovided at the upper end thereof with an integral substantiallyrectangular 100 flange 19, each of the four corners of said flangehaving a passage 20 therethrough to accommodate the shanks of screws 21which respectively take into tapped openings provided in the integralflange 22 of a metal casting designated in 105 general by the numeral23. Cup 10 is provided at its upper end with a relatively largecylindrical bore 24 and has a relatively smaller cylindrical bore 25opening to bore 24. Bore 25 is closed by the bottom wall 26 of cup10,-the passage 13 110 being drilled through the side wall 10 of cup 10in a line substantially flush with said bottom wall 26. Passage 17 islikewise drilled through the wall of cup 10 in a line substantiallyflush with the shoulder 27 formed between bores 24 and 25.

The exterior of side wall 10 of cup 10 is preferably flat to accommodatethe flat face 28 of a metal casting 29 in which the other portions l4,l5 and 16 of the timing passage are drilled in the manner illustrated.The upper end of portion 15 is preferably enlarged as illustrated toreceive with a close fit the cylindrical lower end 30 of a sort ofneedle-valve 30 whereby the adjacent end of portion 16 of the timingpassage may be closed or opened to the desired degree to control therate of flow of the mercury. For purposes of such adjustment valve 30 isprovided with a threaded portion 30 which cooperates with the tappedportion 29 of casting 29,-the upper end of said valve being providedwith a kerf 30 to provide for adjustment thereof by means of ascrew-driver.

Surrounding the upper end of valve 30 is an externally threaded bushing31 which cooperatively engages a threaded portion of said casting 29,theupper end of said bushing being preferably provided with a flange 31 ofpolygonal contour to permit tightening thereof by means of a wrench-andthe inner end of said bushing being adapted to compress a suitable glandor packing ring 32 whereby leakage of mercury around valve 30 isprevented. An internally threaded cap 33 is provided for attachment tothe upper end of casting 29, said cap being knurled or otherwiseroughened as indicated at 33*- to facilitate manipulation thereof.

Casting 29 is rigidly attached to cup 10 in any suitable manner, as bymeans of a number of screws the shanks of which take into tappedrecesses in the wall of cup 10, as indicated in dotted lines at 34 inFig. 2. A relatively thin sheet of fiber 35 is preferably clampedbetween cup 10 and casting 29, said sheet having openings alined withthe ends of portions 13, 14 and 17, 16 of the timing passage and havingopenings to accommodate the aforementioned attaching screws 34. Saidsheet is also preferably coated on each side with Bakelite varnish orshellac whereby a liquid tight seal between cup 10 and casting 29 isprovided upon assembly of said parts. The countersunk recesses 34* (Fig.2) which accommodate the heads of screws 34 are preferably filled with asuitable wax.

The molded Bakelite-graphite piston 11 is provided with a centralcylindrical passage 36 within which a bushing 37 is adapted to fitclosely. The upper end of piston 11 is provided with an annularupstanding flange or projection 38,

1 and the horizontal flange 39 of bushing 37 fits against said flange 38whereby an annular recess 40 is provided at the upper end of saidpiston. Extending upwardly along the walls of passage 36 and openinginto recess 40 are a plurality of recesses 41 (preferably three innumber) the lower ends of which open to a countersink or recess 42 atthe lower end of piston 11.

The lower ends of recesses 41 are partially closed by the overlyingcorners of a nut 43 by which bushing 3'7 is rigidly attached to thepiston. It will be noted, however, from Fig. 1, that the walls ofrecesses 41 diverge outwardly toward the lower ends thereof. Thearrangement is pref erably such that the cleared openings at the lowerends of recesses 41 are substantially equal in area to the threeopenings 44 which penetrate flange 39 in line with the annular recess 40aforementioned,said openings 44 extending through segmental projections39 which have smooth fiat upper surfaces and against which thedownwardly offset annulus 45 of a valve member 45 is adapted to normallyseat under the bias of a coiled compression spring 46.

After the bushing 37 has been secured to piston 10 by nut 43 the screwthreads at the lower ends of said elements are preferably upset ordistorted, as by means of a prick punch or the like to avoid anypossibility of loosening thereof. The lower end of bushing 37 isprovided with an inwardly extending annular flange 47 which leaves acircular opening 48 against the upper edge of which a steel ball valve49 is adapted to seat. Ball 49 is preferably formed separately in a wellknown manner to insure concentricity of its surface, and the same isthen attached, as by spot welding, to the lower end 50 of a valve stem51. Slidable upon stem 51 and adapted to bear against ball 49 is amember 52 which may conveniently consist of a section of tubular gearstock of the required size,--the spaces 52 betweeen the teeth thereofforming suitable passages or clearance for the fiow of mercury when ball49 is displaced from its seat. The teeth are cut away for a portion ofthe length of member 52 whereby a cylindrical guide and a horizontalseat or abutment are provided for the lower end of a coiled compressionspring 53.

Spring 53 is initially placed under the required degree of compressionby a hollow metal collar or ferrule 54 which is seated upon a suitablehorizontal shoulder provided upon the interior of bushing 37 (see Fig.1), the horizontal flange 39 of said bushing being prick-punched at aplurality of points 55 as shown to rigidly and permanently secure saidferrule in position.

The valve stem 51, which extends through an opening 45 in valve member45, is provided with a relatively long threaded portion 51 at its upperend. Threaded onto said upper end is a nut 56 the reduced cylindricallower end 56 of which forms a guide and abutment for the upper end ofspring 46. The end 51 of stem 51 is then threaded into a tapped openingin the lower end of a lever 57,a lock washer 58 being preferablyinterposed between nut 56 and said lever.

The upper end of lever 57 is pivoted by means of a pin 59 to a lever 60at a point above the longitudinal center line of the latter, lever 60being composed of relatively thick metal stock and being cut away fromthe line 60 toward its outer end to provide adequate clearance for lever5'7. As shown in Fig. l, lever 60 is provided with an opening 60 havingfiat sides 60* and 60 to receive with a drive fit the correspondinglyshaped portion 61 of an actuating shaft 61. As shown in Fig. 6 theportion 61 of said shaft is adapted to bear within a ferrule or bushing62 of stainless steel or other suitable hard metal,-said bushing 62being shrunk within a suitable bearing opening in the casting 23.Bushing 62 projects in wardly an appreciable distance from the wall ofcasting 23 and the lever 60 is provided with an annular recess 60concentric with shaft 61, said recess being adapted to provide a closefit over the end of said bushing 62. A bearing is thus provided for theright-hand end of shaft 61 to effectually prevent leakage of mercurythereabout, Bushing 62 likewise cooperates with the A otherpartsillustrated to prevent endwise movement of shaft 61 toward theright.

As will be understood, shaft 61 is placed in position through a suitableopening 63 in the lefthand wall of casting 23, said opening likewiseacting as a bearing for the relatively large end 61 01 said shaft.Casting 23 is provided with a countersink or recess 64 to receive adished metal plug or disk 65 of well known form, said plug beingflattened as illustrated to bite into the walls of recess 64 wherebyafiuid-tight closure is provided, said plug also acting as an abutmentto restrain shaft 61 against endwise movement towardthe left.

As best illustrated in Fig. 1 a thin sheet of fiber 66 is interposedbetween the flanges 19 and 22 of cup 10 and casting 23, respectively,said sheet being coated on both sides thereof with Bakelite varnish orshellac to insure a fluid-tight connection between these parts whenassembled. Lock washers 21 are preferably associated with screws 21 toprevent accidental loosening of the latter.

It will be seen that by the construction and arrangement of parts asaforedescribed, the required volume of mercury may be placed within thetiming device at the factory and all of the parts may be shipped infully assembled relation since there is no possibility of the mercuryescaping during transit or in handling of the device. This feature is ofconsiderable importance from a commercial viewpoint, inasmuch as thedevice may be adjusted at the factory to provide a given timing functionof the device and there is no possibility of any accidental variation ofthe adjustment such as might ensue if any of the parts were required tobe disassembled prior to installation of the device.

The casting 23 is provided with a perforated bracket or projection suchas 67 whereby the same may be attached to a suitable support,-a web orflange 68 being formed integrally with said bracket and the main body ofthe casting to strengthen the same.

In Fig. 4 we have illustrated an adaptation of our timing device tocontrol the closing and reopening of a pair of normally open contacts 69and 70. Contact 69 is carried by a flexible conductive strip 69 which isattached to an L- shaped metal bracket 69 ,--the latter being in turnsecured by a screw '71 to an insulating plate or panel '72, a nut 78acting to clamp the aforementioned parts to said panel, and a nut '74serving as means for attachment of a terminal wire.

Contact 70 is likewise carried by a conductive strip '70 which isattached to member 70 ,-the latter having a threaded shank whereby thesame may be clamped to panel 72 as by means of a nut 75,-the nut 76serving as means for attachment of a terminal wire. The contact partsaforedescribed are preferably enclosed within or partially surrounded bythe walls of a recessed insulating block 77, which is attached in anysuitable manner to the member 70 One end of strip '70 extends outwardlybeyond the contact 70 and block 77.

The projecting end of shaft 61 is provided with a flat portion 61 and alever '78 is non-rotatably fitted thereonto,a set screw '79 or the likebeing employed to prevent movement of said lever endwise of the shaft.Secured to lever 78, as by v means of screws 80 is a block or plate 81of in- The free end of lever 78 has pivoted thereto at 82 a lever 83,the upper end 83 of which extends through a perforation 84 in an endwisepivoted stud or block 85 carried by a lever 86, which is preferably castto the form illustrated. Attached at one end to lever 86, as indicatedat 87, and at its other end to a lug 88 attached to lever 83 is a coiledtension spring 89 whereby lever 83 is normally biased'upwardly towardlever 86. The end 83 of lever 83 is threaded and a nut 90 is adjustablycarried thereon beneath the block 85 whereby the upward movement oflever 83 is limited as shown when lever 86 is in its normal or lowerextreme position. Nut 90 thus provides simple means for adjustment ofthe initial spacing of insulating block 81 with respect to the end ofstrip '70.

Lever 86 is secured as by means of clamping plate 91 and screws 92 tothe squared end 93 of an oscillatable shaft, said shaft being supportedupon panel '72 as by means of suitable bearing brackets one of which isshown at 94. It may be assumed that shaft 93 is manually operable in aclockwise direction for directly or indirectly effecting closure of aplurality of contacts (not shown) to provide starting connections for anelectric motor at will, whereas contacts 69 and '70 when closed areadapted to provide for completion of the running connections of themotor; said contacts 69 and '70 in practice being preferably adapted tocontrol the energizing circuit of an accelerating relay of well knownform (not shown), although if desired the same might be arranged todirectly control completion of the running circuit.

1 Thus in operation the shaft 93 is moved in a clockwise direction toeffect completion of a given electric circuit and such motion tends(through spring 89) to effect immediate upward. movement of levers 83and 78 with consequent closure of contacts 69 and '70. The dash-pottiming device aforedescribed, however, retards upward movement of lever'78, so that energy is stored in spring 89. Piston 11 therefore movesdownwardly within cup 10 at a rate determined by the setting ofneedle-valve 30, and the arrangement is such that just prior toengagement of contact 70 thereof shown or indicated in Fig. 4 may be ef-A fected quite rapidly, inasmuch as the ball valve 49 is then removedfrom its seat (see Fig. 3) and the mercury flows freely through thespace between member 45 and the flange 39 of bushing 37 downwardlythrough the interior of the latter. A certain amount of the mercury maylikewise flow in a reverse direction through portions 17, 16, 15, 14 and13 of the timing passage, and during a portion of the upward movement ofthe piston the recesses or passages 41 will likewise be open.

As aforeindicated needle-valve 30' may be accurately adjusted as desiredto provide for variation of the timing period through a very wide range.

During manufacture of the timing device herein described the moldedpiston and the molded cup with which it is to be associated are tested,and the dimensions thereof changed if necessary, to insure a veryaccurate sliding fit of the former within the latter, with an allowanceof, say, one and one-half thousandths of an inch minimum to twothousandths of an inch maximum clearance between the respectivediameters. The piston is thereafter kept with the respective cup towhich it has been fitted.

While we have described the cup and piston as being molded from asuitable composition of Bakelite and graphite, it is to be understoodthat under certain conditions the graphite may be omitted from themolding compound without sacrificing any of the novel resultscontemplated herein.

What we claim as new and desire to secure by Letters Patent is:

i. In a dash-pot timing device, in combination, a cup and a piston, thelatter having a close sliding fit within the former, and said elementsbeing composed of materials whereby the same have a relatively lowcoeflicient of friction relatively to each other, a body of mercuryinterposed between said elements, and means providing a by-pass forventing said body of mercury and adapted to control the rate of movementof one of said elements with respect to the other.

2. In a mercury dash-pot timing device for electric switches, incombination, a cylinder and a piston, the former having a close slidingfit within the latter, said cylinder and piston each being molded from acomposition comprising graphite and the material sold under the tradename of Bakelite whereby said parts have a relatively low coeiiicient offriction with respect to each other, a body of mercury interposedbetween said elements, means including an adjustable by-pass for ventingsaid body of mercury and adapted to control the rate of movement of saidpiston with respect to said cylinder, a. hollow cap having amercury-tight connection with the open end of said cylinder, anactuating element penetrating said cap and having a mercury-tightbearing connection with the latter, and means providing an operativeconnection between said actuating element and said piston.

3. In a dash-pot timing device for electric switches and the like, incombination, a cup and a piston each formed of a composition comprisingBakelite and graphite, said cup having a venting passage associatedtherewith, and a body of mercury within said cup and cooperating withsaid venting passage to control the rate of movement of said pistonrelatively to said cup.

4. In a dash-pot timing device for electric switches and the like, incombination, a cup and a piston each formed of a composition comprisingBakelite and graphite, said cup having a venting passage associatedtherewith, a body of mercury within said cup and cooperating with saidventing passage to control the rate of movement of said pistonrelatively to said cup, and means for varying the size of said ventingpassage whereby the aforementioned rate of movement may be definitelyvariedat will.

5. In a dash-pot timing device for electric switches and the like, incombination, a cup and a piston therefor each being molded from acomposition consisting of Bakelite and graphite, said cup having aventing passage associated therewith, a body of mercury within said cupand cooperating with said venting passage to initially control the rateof downward movement of said piston relatively to said cup, and valvemeans associated with said piston and said cup and operableautomatically to additionally vent said body of mercury whereby aportion of the downward movement of said piston is accelerated.

6. In a dash-pot timing device, in combination, a cup, a piston slidablymovable within said cup, said piston having a central passagetherethrough, a ball valve at the lower end of said passge andspring-biased to normally eifect closure of the latter, said pistonhaving a plurality of passages extending therethrough and arrangedaround said first-mentioned passage, a valve member spring-biased toeffect closure of said last-mentioned passages, a body of mercury withinsaid cup, and means providing an adjustable venting passage independentof all of the passages aforementioned for initially controlling the rateof downward movement of said piston relatively to said cup, said valvemember being operable automatically at a given point in the travel ofsaid piston to open said second-mentioned passages whereby the rate ofdownward movement of said piston is accelerated.

7. A mercury dash-pot timing device for electric switches and the like,comprising a cup and a piston closely fitted within said cup andslidable with respect thereto, said piston having a plurality ofpassages extending from end to end thereof, spring-pressed valve membersat the upper and lower ends of said piston for normally closing therespective passages, means directly connected with the lower valvemember and operable to effect downward movement of said piston, a bodyof mercury within said cup beneath the piston to resist downwardmovement of the latter, means providing an adjustable by-pass for saidbody of mercury whereby the initial rate of downward movement of saidpiston may be definitely controlled, and a shoulder on said cup to beengaged by the upper valve member to open the latter at a given point inthe downward movement of said piston, whereby such downward movement isaccelerated.

8. A liquid dash-pot timing device for electric switches and the like,comprising a cup and a piston closely fitted within said cup andslidable with respect thereto, said piston having a plurality ofpassages extending from end to end thereof, spring-pressed valve membersat the upper and lower ends of said piston for normally closing therespective passages, means directly connected with the lower valvemember and operable to eifect downward movement of said piston, a bodyof liquid within said cup beneath the piston to resist downward movementof the latter, means providing an adjustable by-pass for said body ofliquid whereby the initial rate of downward movement of said piston maybe definitely controlled, and a shoulder on said cup to be engaged bythe upper valve member to open the latter at a given point in thedownward movement of said piston, whereby such downward movement isaccelerated.

9. In a dash-pot timing device for electric switches and the like, incombination, a molded composition cup and a molded composition pistonclosely fitted within said cup and slidable with respect thereto, ametal member secured to a side wall of said cup, said metal memberhaving a venting passage extending therethrough and through said sidewall of the cup at vertically spaced points in the latter, a body ofmercury within said cup and displaceable through said passage bydownward movement of said piston, a valve member carried by said metalmember and adjustable to vary the size of said venting passage wherebythe rate of downward movement of said piston may be definitelycontrolled, a chambered metal cover member secured to the open end ofsaid cup,,a horizontally disposed shaft bearing within openings inopposed walls of said cover member and having an end portion projectingoutwardly from the latter, means associated with said bearing openingsto prevent leakage of mercury from said chamber, actuating means forsaid shaft, and a leverage connection between the latter and saidpiston.

10. In a device of the character described, in combination, a pistonmolded from a composition of Bakelite" and graphite, said piston havinga relatively large central passage extending longitudinally therethroughand a plurality of passages surrounding said central passage and openingthereinto, a hollow metal bushing having a body portion adapted to fitclosely within said central passage to separate the second-mentionedpassages therefrom, said body portion having an inwardly extendingannular flange providing a valve opening at the lower end thereof, avalve member located within said body member and spring-biased intoengagement with said valve opening, said bushing having a horizontalflange and said piston having an upstanding annular flange whereby anannular recess is provided between said parts, said second-mentionedpassages being arranged to communicate with said recess, said horizontalflange having a plurality of openings therethrough, and a metal disksuperimposed upon said horizontal flange and springbiased intoengagement with the latter to normally close said openings.

11. In a device 01' the character described, in combination, a pistonmolded from a composition of Bakelite" and graphite, said piston havinga relatively large central passage extending longitudinally therethroughand a plurality of passages surrounding said central passage and openingthereinto, a hollow metal bushing having a body portion adapted to fltclosely within said central passage to separate the second-mentionedpassages therefrom, said body portion having an inwardly extendingannular flange providing a valve opening at the lower end thereof, avalve member located within said body member and spring-biased intoengagement with said valve opening, said bushing having a horizontalflange and said piston having an upstanding annular flange whereby anannular recess is provided between said parts, said second-mentionedpassages being arranged to communicate with said recess, said horizontalflange having a plurality of openings therethrough, and a metal disksuperimposed upon said horizontal flange and spring-biased intoengagement with the latter to normally close said openings, saidball-valve and said disk being movable independently of each other toeffect opening of the respective passages controlled thereby.

12. In a dash-pot timing device for electric switches and the like, incombination, a molded composition cylinder having a closed lower end, apiston closely fltted within said cylinder and slidable with respectthereto, a cast metal member secured to one side of said cylinder, saidmember having a venting passage extending therethrough and communicatingwith the interior of said cylinder at vertically spaced points in thelatter, a body of liquid within said cylinder and displaceable throughsaid passage by downward movement of said piston, adjustable meanscarried by said member and associated with said venting passage forvarying the size of the latter whereby the rate of downward movement ofsaid piston may be controlled, a chambered metal cover member secured tothe open end of said cylinder in liquid-tight relation thereto, saidcover member having a pair of bearing openings formed in the opposedwalls thereof, a horizontally disposed shaft insertable endwise intosaid bearing openings and having an end portion extending outwardly fromsaid cover member, means comprising a pair of metal members respectivelyassociated with said bearing openings and with said shaft to restrainthe latter against endwise movement in either direction and to insureagainst leakage of liquid from said cover member, actuating meansconnected with the projecting end of said shaft, and a leverageconnection between the latter and said piston.

13. In a device of the character described, in combination, a piston,said-piston having a relatively large central passage extendinglongitudinally therethrough and having a plurality of passagessurrounding said central passage and opening thereinto, a hollow bushinghaving a body portion adapted to flt closely within said central passageto separate the second-mentioned passages therefrom, said body portionhaving an inwardly extending annular flange providing a valve opening atthe lower end thereof, a valve member located within said body memberand spring-biased into engagement with said valve opening, said bushinghaving a horizontal flange and said piston having an upstanding annularflange whereby an annular recess is provided between said parts, saidsecond-mentioned passages being arranged to communicate with saidrecess, said horizontal flange having a plurality of openingstherethrough, a metal disk superimposed upon said horizontal flange andspring-biased into engagement with the latter to normally close saidopenings, a cylinder -within which said piston is slidably mounted, abody of liquid interposed between the closed lower end of said cylinderand said piston, means additional to the passages aforementioned forcontrollably venting said liquid upon initial downward pressure on saidpiston, and means located in the path of said metal disk for displacingthe latter at a given point in the downward movement of said piston,whereby said liquid is further vented to decrease the resistance tomovement of said piston.

14. In a device of the character described, in combination, a piston,said piston having a relatively large central passage extendinglongitudinally therethrough and having a plurality of passagessurrounding said central passage and opening thereinto, a hollow bushinghaving a body portion adapted to flt closely within said central passageto separate the second-mentioned passages therefrom, said body portionhaving an inwardly extending annular flange providing a valve opening atthe lower end thereof, a valve member located within said body memberand spring-biased into engagement with said valve opening, said bushinghaving a hori zontal flange and said piston having an upstanding annularflange whereby an annular recess is provided between said parts, saidsecond-mentioned passages being arranged to communicate with saidrecess, said horizontal flange having a given point in the downwardmovement of said piston whereby said liquid is further vented todecrease the resistance to movement of said piston, the means foractuating said piston including means for upwardly displacing said firstmentioned valve member to provide a relatively large venting passage,whereby said piston may be quickly reset to its upper extreme position.

EDWIN x. SCHMIDT. WILLIAM C. STEVENS.

